1. Field of the Invention
The present invention relates to a single-ended swash plate compressor for use in automotive vehicles and the like.
2. Description of the Related Art
Swash plate compressors, in which a plurality of cylinder bores are disposed parallel to a drive shaft in a peripheral portion of a cylinder block, with piston assemblies housed in the cylinder bores, the piston assemblies being reciprocated by a swash plate which rotates together with the drive shaft so as to compress a refrigerant gas, are in general use as compressors for conventional automotive air-conditioners. Moreover, double-ended swash plate compressors, which include double-headed piston assemblies in which compression pistons are formed on both ends of piston rods and a compression action is performed at both the front end and the rear end of the piston bores, are often used. However, when using carbon dioxide (C02) as a refrigerant as an alternative to chloro fluorocarbons, there are cases where single-ended swash plate compressors are used.
Generally-known conventional single-ended swash plate compressors include single-headed piston assemblies in which compression pistons are formed on one end of the piston rods only and the compression action is performed at one end of the piston bores, for example, the rear end only.
The fixed-capacity single-ended swash plate compressor shown in FIG. 13 is a known example of such a swash plate compressor.
In the figure, the outer shell 201 of the compressor is formed by joining a front housing 201b to the front end of a cylinder block 201a, forming a swash plate chamber 202 within. A cylinder cover 203 functioning as a rear housing having a discharge chamber 203a and an intake chamber 203b therein is joined to the rear end of the cylinder block 201a by means of a valve plate 204. An intake port 205 for receiving intake gas from an external refrigerant circuit (not shown) is disposed in a side wall of the cylinder cover 203 and is connected to the intake chamber 203b. A drive shaft 206 is disposed in a central portion of the outer shell 201 of the compressor and is rotatably supported by radial bearings 207. A plurality of cylinder bores 208 are formed in the cylinder block 201a parallel to the drive shaft 206 and equidistantly spaced in a circle of fixed circumference centered on the drive shaft 206. Consequently, a cylinder assembly is formed by the cylinder block 201a. Piston assemblies 209 each comprise a piston rod 209b and a single-headed piston 209a formed on the rear end of the piston rod 209b. A single-headed piston 209a is housed within each of the cylinder bores 208 so as to be free to slide and reciprocate.
A swash plate 210 is secured to the drive shaft 206 within the swash plate chamber 202 so as to rotate together with the drive shaft 206, the pistons 209a being engaged by the swash plate 210 by means of shoes 211. Furthermore, a thrust bearing 214 is disposed at the front end of a boss portion 210a of the swash plate 210, that is to say, between the boss portion 210a and the front housing 201b, thrust loads acting on the swash plate 210 being supported by the thrust bearing 214.
Discharge holes 204a connecting each of the cylinder bores 208 to the discharge chamber 203a and intake holes 204b connecting each of the cylinder bores 208 to the intake chamber 203b are disposed in the valve plate 204. An intake valve-forming plate 212 integrally formed with a plurality of intake valves 212a for controlling the opening and closing of each of the intake holes 204b is interposed between the valve plate 204 and the cylinder block 201a, and a discharge valve-forming plate 213 integrally formed with a plurality of discharge valves 213a for controlling the opening and closing of each of the discharge holes 204a is interposed between the valve plate 204 and the cylinder cover 203.
Gas passages 215 are disposed in the cylinder block 201a in the spaces between the plurality of cylinder bores 208, the swash chamber 202 being connected to the intake chamber 203b by means of the gas passages 215, so that blowback gas flowing into the swash chamber 202 during the process of compression by the pistons 209a is expelled to the intake chamber 203b.
Moreover, 216 is a retainer, 217 is a discharge port, and 218 is a bolt joining the cylinder block 201a, the front housing 201b, and the cylinder cover 203 together.
When a single-ended swash plate compressor constructed in the above manner is activated, intake gas is directed from the external refrigerant circuit through the intake port 205 into the intake chamber 203b. Then, the refrigerant gas is taken from the intake chamber 203b through the intake holes 204b and intake valves 212a into the cylinder bores 208 and is compressed by the pistons 209a. The compressed refrigerant gas is expelled through the discharge holes 204a and the discharge valves 213a to the discharge chamber 203a and is discharged through the discharge port 217 to the external refrigerant circuit.
In a single-ended swash plate compressor constructed in the above manner, the front ends of the pistons 209a (left side in figure) are exposed to the swash chamber which is at intake pressure, and at the same time the rear ends of the pistons 209a are exposed to the cylinder bores 208 which are filled with compressed refrigerant gas, Thus, the internal pressure (intake pressure) of the swash chamber 202 acts on the front end surface of each of the pistons 209a, and the internal pressure of the cylinder bores 208 acts on the rear end surface of each of the pistons 209a. FIG. 14 is a graph explaining the conditions in one piston and shows the changes in the internal pressure Pc in the swash plate chamber 202 and the changes in the internal pressure Pb in the cylinder bore 208 relative to the rotational angle of the swash plate 210 (in degrees). As shown in this diagram, the internal pressure Pc in the swash plate chamber 202 always remains at a practically constant low pressure, that is at the intake pressure, but the internal pressure Pb in the cylinder bore 208 fluctuates periodically between a low intake pressure and a high discharge pressure depending on the rotational angle of the swash plate 210.
Now, thrust loads from the front end towards the rear end act on the front end surfaces of the pistons 209a, and thrust loads from the rear end towards the front end act on the rear end surfaces of the pistons 209a. Thus, the thrust load acting on the thrust bearing 214 is given by the sum of these loads acting on the pistons 209a.
FIG. 15 is a graph explaining the axial load, and the vertical axis shows the thrust load, the direction from the rear end towards the front end being taken as positive. The number of pistons 209a has been taken to be six and the loads acting on all six pistons have been totalled. In FIG. 15, Ff indicates the thrust load acting from the front end towards the rear end due to the internal pressure in the swash chamber 202. Fr indicates the thrust load acting from the rear end towards the front end due to the internal pressure in the cylinder bores 208. Ft indicates the total load resulting from Ff and Fr. Since Ft is the sum of all of the loads acting on a plurality of pistons (in this case six), the amplitudes and periods of the fluctuations are small compared to those of the internal pressure in the single cylinder bore 208 shown in FIG. 14.
Now, as can be understood from FIGS. 14 and 15, because the difference between the internal pressure Pb in the cylinder bores 208 and the internal pressure Pc in the swash plate chamber 202 is great, the difference between the thrust load Ff acting from the front end towards the rear end and the thrust load Fr acting from the rear end towards the front end is great, making the overall total thrust load Ft a large unbalanced load from the rear end towards the front end. This unbalanced load is transmitted through the shoes 211 to the swash plate 210 and is supported by the thrust bearing 214 disposed at the front end of the boss portion 210a of the swash plate 210 so as to support the thrust load from the swash plate 210.
Thus, in a conventional fixed-capacity single-ended swash plate compressor, because compression is performed on only one side of the swash plate, the load acting on the thrust bearing 214 disposed at the front end of the boss portion 210a of the swash plate 210 is great. In particular, the working pressure when carbon dioxide is used as the refrigerant is greater than when chloro fluorocarbons or the like are used, which tends to shorten the working life of the thrust bearing 214 disposed at the front end of the swash plate 210, and a thrust bearing 214 with a high load rating is required to prevent this. However, the problem is that by using a thrust bearing 214 with a high load rating, the size of the thrust bearing 214 at the front end is increased, in turn leading to increases in the size and weight of the compressor.